This invention relates to relay circuits and, more particularly, to a relay circuit for controlling an AC voltage which utilizes a pair of photo sensitive silicon controlled rectifiers, hereinafter referred to as photo-SCRs, as the active switching elements and which includes means for triggering the photo-SCRs into conduction only when the value of the AC voltage being controlled is within predetermined limits of zero volts.
A photo-SCR is a type of semiconductor control switch which may be triggered into conduction by a gate signal in a manner similar to the action of the well known conventional silicon controlled rectifier (SCR). However, the photo-SCR differs from the conventional SCR in that the photo-SCR may be triggered into conduction by radiant photo energy incident on the photo-SCR, as well as by conventional means of applying an external electrical signal to the photo-SCR gate. The radiant energy in turn may be generated by a light source such as a light emitting diode (LED). Since SCR devices conduct only in one direction, two photo-SCRs connected in inverse-parallel may be employed for full wave control in AC circuits.
A number of circuits have been devised in the prior art to provide an AC relay consisting of a pair of inverse-parallel connected photo-SCRs triggered into conduction by LED light sources which are optically coupled to the SCRs through a transparent dielectric path. The LEDs are in turn energized by an input signal. The light output from the LED light source impinges on the photo-SCRs, causing them to conduct and complete an output circuit between an AC power source and a load. Such relay circuits are widely used for the control of AC loads by computer generated logic signals such as are found in microcomputer based control systems. The optical coupling between the input signal circuit and the load circuit provides excellent electrical isolation between the logic signals and the AC loads. In addition, the photo-SCR relay circuit thus described requires few components for its implementation. This circuit simplicity permits the use of hybrid circuit construction techniques for the manufacture of these devices. As is well known to those skilled in the art, such construction techniques may employ ceramic substrates and lead frames to support the semiconductor chips with the resultant assembly being molded in plastic. The use of these construction techniques provides a solid state AC relay circuit which may be inexpensively mass produced in a very small package size.
Although the prior art photo-SCR AC relay circuits have gained widespread use, they possess several limitations. For example, it is well known that for most applications it is desirable to have an AC relay which switches on only at those points in time when the value of the AC voltage being controlled is substantially equal to zero. This eliminates the transient problems which occur if the relay is switched on at an instant when the AC voltage being controlled has a substantial value. This type of circuit action is known to those skilled in the art as zero voltage switching, although the actual switching action may occur within a range of voltages around zero.
Several circuits have been devised in the prior art to provide zero voltage switching for AC relays using photo-SCRs. However, such circuits possess several limitations. Many of these zero voltage switching circuits employ a large number of components to achieve acceptable performance. These circuits have not gained widespread use because they negate the ease of construction, small size and low cost advantages of the basic nonzero voltage switching photo-SCR circuits. In an effort to provide a zero voltage switch circuit using a minimum number of components, prior art designs have employed a transistor having its collector and emitter connected to the photo-SCR gate and cathode respectively. The transistor base is connected through a series current limiting resistor to the photo-SCR anode. The photo-SCR anode-cathode voltage is thus used to control the transistor. The object of this circuit is to use the transistor to clamp the gate-cathode circuit of the photo-SCR whenever the voltage across the photo-SCR anode-cathode circuit exceeds a predetermined value. Clamping the photo-SCR gate-cathode circuit prevents the photo-SCR from being triggered into conduction, even if there is radiant photo energy incident on the photo-SCR.
Unfortunately, the SCR gate clamping circuit thus described has the effect of significantly increasing the level of irradiance required to trigger the photo-SCR into conduction. This level of irradiance is a measure of the intensity of the photo energy incident on the photo-SCR that is required to trigger the SCR into conduction. A significant increase in the level of irradiance requires a corresponding increase in the light output of the light source used to trigger the photo-SCRs into conduction. In the case of an LED light source, an increase in light output is accomplished by an increase in the LED current supplied by the input signal source. Accordingly, it has been found that prior art AC relay circuits employing the zero voltage switching configuration described heretofore require high levels of input signal current for their operation. These high levels of input current preclude the prior art zero voltage switching AC relay circuits from being directly controlled by computer generated logic signals. As indicated previously, a major application of these AC relay circuits is as a direct interface between computer logic and AC loads. Computer logic circuits are limited in the amount of current they can supply to drive the input of the AC relay circuit. By exceeding these logic current levels, additional amplifiers and power drivers are required to interface the prior art relay circuits to the computer logic. Additional power supplies may also be required to supply the increased current levels.
The requirement for higher levels of light output in prior art circuits causes an additional disadvantage. It is well known to those skilled in the art that LEDs exhibit a phenomenon known as output degradation. That is, the light output of an LED decreases with time. The rate of degradation of light output is a function of the LED current level, with higher LED currents producing faster degradation. Thus an AC relay circuit requiring a high level of light output and therefore a high level of LED current exhibits a rapid degradation of performance.
It is therefore an object of the present invention to provide a new and improved zero voltage switching AC relay circuit.
It is another object of the present invention to provide an improved zero voltage switching AC relay circuit which does not require high levels of input current for operation.
It is still another object of the present invention to provide a simple and inexpensive zero voltage switching AC relay circuit which may be constructed with a minimum number of components.